1. Field of the Invention
The present invention relates to resonant tunneling devices, and more particularly to three terminal resonant tunneling transistors.
2. Description of the Related Art
A resonant tunneling device (RTD) typically has an active region with a double barrier structure for resonant tunneling of electrons. The double barrier structure includes a well region bounded on opposite sides by two barrier regions which contact the well region. Electrons are supplied to the active region by two electrode layers, one serving as an emitter and the other as a collector. The electrode layers are placed on opposite sides of the well region in contact with the barrier regions. The well region has at least one bound quantum energy state for electrons.
A resonant tunneling transistor (RTT) is an RTD with the addition of a control electrode, such as a base contact, which supplies voltage or current to the RTD's active region. Charge is added or removed from the quantum well region through the control electrode, changing the voltage differential across the barrier region. This modulates the bound energy states, which in turn controls the emitter-to-collector voltage at which tunneling occurs.
FIG. 1 is an energy band diagram for one exemplary RTT structure described in B. Jogai and K. L. Wang, Applied Physics Letter, Vol. 46, Pages 167-168, (1985), which employs an active region consisting of a GaAs quantum well region 2 and AlGaAs barrier regions 3 and 4. The minimum conduction band energy level (Ec) and the maximum valence band energy level (Ev) are shown for this type I band alignment. Ec for the quantum well region 2 is less than Ec for AlGaAs barrier regions 3 and 4, while Ev for the well region 2 is greater than Ev for the barrier regions 3 and 4. Ec for GaAs emitter 6 and GaAs collector 8 regions on opposite sides of barriers 3 and 4 are approximately equal to one another and approximately at the same level as Ec for the well region 2.
In operation, electrons are provided by the emitter 6 and flow towards the first barrier region 3. To pass over this barrier the emitter 6 electrons must normally be at an energy above the barrier Ec. Resonant electron tunneling is a phenomenon in which the electrons can pass through the first barrier region 3 upon reaching the energy of a bound energy state in the well region 2 which is less than Ec of the first barrier region 3. Since the well region 2 is at a lower energy level than the barrier regions 3 and 4, the electrons that tunnel into the well region 2 accumulate there at an energy level above the well Ec. Conversely, holes flowing between the emitter 6 and the collector 8 accumulate in the well region 2 at an energy level below Ev. The well region 2 can therefore be a well for both holes or electrons.
A base contact (not shown) is made to the RTT's quantum well, injecting electrons (control electrons) into the quantum well to occupy bound energy states and manipulate the tunneling action. The problem with this base contact is that, if the barriers are thin enough to allow for tunneling of emitter electrons, the control electrons provided by the base also tunnel through the thin barriers. This results in a loss of control electrons from the well region, causing excessive base-to-collector leakage current as the base potential is increased, shorting the base-collector junction.
Another RTT structure is the quantum well excited state tunneling transistor (QuESTT). The QuESTT overcomes the problem of base leakage by lowering the Ec of the quantum well layer to narrow the band gap at the quantum well. As a result, the bound energy states in the quantum well are lowered and control electrons in the quantum well require greater energy to tunnel out. A greater number of control electrons are therefore available in the quantum well, potentially solving the problem of base leakage. The QuESTT is described in J. N. Schulman and M. Waldner, Journal of Applied Physics, Vol. 63 , pages 2859-2861, (1988).
In operation, electrons are injected into the quantum well of the QuESTT from its base, modulating the bound energy states. By making the higher bound energy states equal to the emitter electron energy level, tunneling of emitter electrons can be controlled. However, attempts to fabricate an operational device have been unsuccessful because of difficulties in forming a contact to a thin quantum well base.
Another approach to creating an RTT, described in M. A. Reed et. al., Applied Physics Letter, Vol. 54, pages 1034-1036, (1989), uses a bipolar analog of the QuESTT. The active region of this device has a type I band alignment and consists of an InGaAs quantum well bounded by AlGaAs barriers, with GaAs collector and emitter regions. The base contact to the well is a heavily doped p-type semiconductor instead of the n-type base of the QuESTT, and the emitter and collector are n-type, creating a bipolar RTT. Since the base is p-type, holes instead of electrons are injected into the quantum well to control the current from the emitter. The addition of In to the quantum well lowers its Ec below the emitter Ec. This reduces the bandgap between Ec and Ev in the well, thus suppressing hole leakage from the well into the emitter.
A problem with this device is that a p-type current control is introduced to the quantum well region, where the densities of electrons and holes are high. The close proximity of electrons in the well's conduction band to the holes in its valance band attracts the electrons to the holes, scattering the electrons. Electron scattering reduces the device's maximum-to-minimum current ratio, diminishing its negative differential resistance (NDR). NDR is the current-to-voltage relationship of an RTT, characterized by a decrease in the emitter-to-collector current as the emitter-to-collector voltage is increased over a particular voltage range. NDR is desirable because maximum and minimum current levels are achieved, making the RTT functional as a switch or an oscillator.